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一般获取请求

nsmanagedobject generics swift

Magoo · 技术社区 · 7 年前

我有一个静态函数 NSManagedObject

NSManagedObject.get(type: MYUser.self, with: ("id", "SomeUserId"), in: context)

extension NSManagedObject {

    static func get<M: NSManagedObject>(type: M.Type, with kvp: (String, CVarArg), in context: NSManagedObjectContext) -> M? {

     guard let name = entity().name else { return nil }
     guard M.entity().propertiesByName[kvp.0] != nil else { Assert("\(name) does not have \(kvp.0)"); return nil }

     let fetchRequest = NSFetchRequest<M>(entityName: name)
     fetchRequest.predicate = NSPredicate(format: "\(kvp.0) == %@", kvp.1)

     do {
            let object = try context.fetch(fetchRequest)
            if let foundObject = object.first { return foundObject }
            return nil
        } catch {
            return nil
        }
    }
}

我想要的语法是

MYUser.get(with: ("id", "SomeUserId"), in: context)

NSFetchRequest<M>(entityName: name)

NSFetchRequest<???>(entityName: name)

提前谢谢

2 回复 | 直到 7 年前

Sweeper 7 年前

如果你不介意的话 MYUser 两次,你可以移除 type 参数并指定类型,以便Swift可以推断 M :

extension NSManagedObject {

    static func get<M: NSManagedObject>(with kvp: (String, CVarArg), in context: NSManagedObjectContext) -> M? {

        guard let name = entity().name else { return nil }
        guard M.entity().propertiesByName[kvp.0] != nil else {
            print("\(name) does not have \(kvp.0)")
            return nil
        }

        let fetchRequest = NSFetchRequest<M>(entityName: name)
        fetchRequest.predicate = NSPredicate(format: "\(kvp.0) == %@", kvp.1)

        do {
            let object = try context.fetch(fetchRequest)
            if let foundObject = object.first { return foundObject }
            return nil
        } catch {
            return nil
        }
    }
}

// usage:
let user: MYUser? = MYUser.get(with: ("id", "SomeUserId"), in: context)

我的用户 两次,我想不出任何解决办法。如果 NSManagedObject 如果是协议,你可以用 Self 在那里。

Magoo 7 年前

基于链接 Passing generic Class as argument to function in swift Martin R

protocol Managed where Self: NSManagedObject { }

extension Managed where Self: NSManagedObject {

static func get(with kvp: (String, CVarArg), in context: NSManagedObjectContext) -> Self? {

    guard let name = entity().name else { return nil }
    guard entity().propertiesByName[kvp.0] != nil else { Assert("\(name) does not have \(kvp.0)"); return nil }

    let fetchRequest = NSFetchRequest<Self>(entityName: name)
    fetchRequest.predicate = NSPredicate(format: "\(kvp.0) == %@", kvp.1)

    do {
        let object = try context.fetch(fetchRequest)
        if let foundObject = object.first { return foundObject }
        return nil
    } catch {
        return nil
    }
}